Your attempt to replace the grep condition with a variable fails because the first parameter to grep is supposed to be a snippet of code, not a string, and not a variable containing a string.

If you want to dynamically create the condition used with grep, you will need to construct a string containing the entire grep command and then evaluate it using eval. Here is a simple example:

my @aFruits = qw(apple bananna orange pear);

#declare a variable for the grep result
my @aSelected;

#define your condition
my $sCondition = q{/^b/ || /^p/};

#construct the grep command and evaluate it
my $sEval = "\@aSelected = grep($sCondition,\@aFruits)";
eval $sEval;

#see the results
print "selected fruits (eval): " . join(',', @aSelected) . "\n";
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Best, beth