Use the e-modifier: 
#!/usr/bin/perl
my %variables = ('$x' => 1);
my $test = '$x $y';

$test =~ s/(\$\w+)/exists $variables{$1} ? $variables{$1} : $1/e;

print $test, "\n";
__END__
1 $y

[download]

Update: Every possible value is substituted (albeit by itself, if not existing as hash-key), if this is too much or discomforting, just split match and substitution...

regards,
tomte

Hlade's Law:

If you have a difficult task, give it to a lazy person --
they will find an easier way to do it.

In reply to Re: Substitutions if hash entry exists by Tomte
in thread Substitutions if hash entry exists by PhosphoricX

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