Well if it's just a matter of printing a message when foo() returns false, you could just add a print inside sub foo { ... } -- but if it's really "a larger set of things" that are needed on a false return, it might be too much to cram into sub foo { ... } (or maybe you don't have the privelege to muck with what "foo()" does).

In which case, you could try defining a second sub, let's call it "foo_report()", have it do whatever is needed when foo() returns false, and make sure that foo_report() also returns false -- and include it in the grep condition:

@array1 = qw/7 2 5 8 0 10/;

@array2 = grep { foo($_) or foo_report($_) } @array1;

print join " ","array2 contains:",@array2,$/;

sub foo { ( $_[0] / 2 > 1 ) }

sub foo_report { 
    print "foo returned false for $_[0]\n";    
    # do lots of other stuff...
    0;  # make sure to return false, for grep's sake
}

__OUTPUT__
foo returned false for 2
foo returned false for 0
array2 contains: 7 5 8 10
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