Please look at your regex. When you do /$foo/, it interpolates $foo to the string '$bar'. This makes your regex try to match end-of-line ($), followed by 'bar'. That can't happen.

If $foo is '\$bar', then you have a different case. You try to match '\$', which is a literal dollar-sign, followed by 'bar'. That works.

Watch:

friday:~ $ perl -mre=debug
$re = '$bar';
print map ">$_\n", grep /$re/, qw( $foo $bar );
compiling RE `$bar'
size 5 first at 1
   1: EOL(2)
   2: EXACT <bar>(5)
   5: END(0)
anchored `bar' at 0 (checking anchored) minlen 3 
Matching `$bar' against `bar'
  Setting an EVAL scope, savestack=12
   1 <$> <bar>            |  1:  EOL


friday:~ $ perl -mre=debug
$re = '\$bar';
print map ">$_\n", grep /$re/, qw( $foo $bar );
compiling RE `\$bar'
size 4 first at 1
   1: EXACT <$bar>(4)
   4: END(0)
anchored `$bar' at 0 (checking anchored isall) minlen 4 
>$bar
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