my @array1=(1,2,3,4,5);
my @array2=(a,b,c,d,e);

@array1=(1,2,a,b,c,d,e,4,5);

my @array1=(1,2,3,4,5);
my @array2=(a,b,c,d,e);
splice @array1, 2, 1, @array2;
print join( ",", @array1 ), "\n";

> perldoc -f splice
    splice ARRAY,OFFSET,LENGTH,LIST
    splice ARRAY,OFFSET,LENGTH
    splice ARRAY,OFFSET
    splice ARRAY
            Removes the elements designated by OFFSET and LENGTH from an
            array, and replaces them with the elements of LIST, if any. In
            list context, returns the elements removed from the array. In
            scalar context, returns the last element removed, or "undef" if
            no elements are removed. The array grows or shrinks as
            necessary. If OFFSET is negative then it starts that far from
            the end of the array. If LENGTH is omitted, removes everything
            from OFFSET onward. If LENGTH is negative, removes the elements
            from OFFSET onward except for -LENGTH elements at the end of the
            array. If both OFFSET and LENGTH are omitted, removes
            everything. If OFFSET is past the end of the array, perl issues
            a warning, and splices at the end of the array.

            The following equivalences hold (assuming "$[ == 0"):

                push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
                pop(@a)             splice(@a,-1)
                shift(@a)           splice(@a,0,1)
                unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
                $a[$x] = $y         splice(@a,$x,1,$y)

            Example, assuming array lengths are passed before arrays:

                sub aeq {   # compare two list values
                    my(@a) = splice(@_,0,shift);
                    my(@b) = splice(@_,0,shift);
                    return 0 unless @a == @b;       # same len?
                    while (@a) {
                        return 0 if pop(@a) ne pop(@b);
                    }
                    return 1;
                }
                if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

my @array1=(1,2,3,4,5);
my @array2=(a,b,c,d,e);

foreach my $x(0..$#array1)
     {
       my $switch;
       $switch='Y' if $x==2;
       if( $switch eq  "Y")
          {
          splice @array1, $x, 1, @array2;
          # ist array1 jetzt hier bereits  'verlängert' ?
          # d.h. der nächste Wert von $array[$x] ist 'a' ?
          # oder wird nur eine Kopie erstellt, die dann nach                  
          # abarbeiten der Schleife gültig wird?
         }

      }

print "'", join(', ', @array), "'\n";

print Dumper \@array;