# my $file1 = `cat file`;
my $file1; {
   open(my $fh, "< file") or die "Unable to open file: $!\n";
   local $/;
   $file1 = <$fh>;
}
$file1 =~ s!^(.*#REPLACE)(.*)$!$1$file2!e;

my $file1 = '/path/to/file/one';
my $file2 = '/path/to/file/two';


open FILE1, $file1 or die $!;
my @content1 = <FILE1>;
close FILE1;

open FILE1, '>'.$file1 or die $!;
foreach my $line (@content1) {
    if ($line =~ m~^(.*#REPLACE)~) {
        open FILE2, $file2 or die $!;
        print FILE1 $1,(<FILE2>);
        close FILE2;
        last;
    }

    print FILE1 $line;
}
close FILE1;