my $file = 'template.txt';
my $foo = 'footext';
my $content = '';

{
    local $/;
    open my $fh, '<', $file or die $!;
    $content = <$fh>;
}

$content =~ s/\$foo/$foo/g;
print $content;

my $content = do{ local (@ARGV,$/) = $file; <> };

no strict;
$txt =~ s/\$([A-Za-z0-9_]+)/${$1}/eg;

s/\$([A-Za-z0-9_]+)/$main::{$1}/eg;

#!/usr/bin/perl

use strict;
use warnings;

my $content = do{ local $/; <DATA> };
our $foo = 'hallo';

no strict;
$content =~ s/\$(\w+)/${$1}/eg;
print $content;

__END__
text text $foo text $foo text 

C:\>perl test.pl
text text hallo text hallo text

#!/usr/bin/perl

use strict;
use warnings;

my $content = do{ local $/; <DATA> };
my $foo = 'hallo';

no strict;
$content =~ s/\$(\w+)/${$1}/eg;
print $content;

__END__
text text $foo text $foo text

C:\>perl test.pl
Use of uninitialized value in substitution iterator at test.pl line 10, <DATA> line 1.
Use of uninitialized value in substitution iterator at test.pl line 10, <DATA> line 1.
text text  text  text

#!/usr/bin/perl

$| = 1;

use strict;
use warnings;

our $test = 123; 
our $MOPS = 'mOpZ';
my $text = 'Ein $test ist gut! #
Oder $MOPS';
no strict;
$text =~ s/\$([A-Za-z0-9_]+)/'$'.$1/eegm;
print $text;

 perl -Mstrict -wle 'my $b = "test"; $_=q{a $b c};s/\$([A-Za-z0-9_]+)/$main::{$1}/eg; print'
Use of uninitialized value in substitution iterator at -e line 1.
a  c