Greetings,
I feel this has more to do with the binding operator  =~ than it does with your entry widgets get method. Here is the perlop snippet that makes me think so. clipped for clarity

...``=~'' binds a scalar expression to a pattern match. ... (If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time.

So if you simply type in "foo" you get a match since the  =~ seems to be treating it as /foo/ by default.
One workaround might be to strip off the "qr/" and trailing "/" prior to the match.
Here is an example of the behavior

#!/usr/bin/perl -w
use strict;

my $string = "foobarbaz";
my $exp = "foo";
if($string =~ $exp){
    print "without /'s works fine";
}elsif($string =~ /$exp/){
    print "with /'s is needed";
}else{
    print "WTF!??";
}
[download]

This outputs

without /'s works fine
[download]

-InjunJoel