It should be a simple substitution:

$a  = "this is a test";
%b = ( is => 'was' );
foreach my $key ( keys %b ) {
   $a =~ s/$key/$b{$key}/g;
}
print $a,"\n";
# output:      thwas was a test
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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain