Why does it need to actually have a zero or a one as the result? If you just check the result of the grep you will know whether the element exists or not:

  #!/usr/bin/perl -w
use strict;
my @a;
my $f;
$a[0] = "a" ;
$a[1] = "c" ;
$a[2] = "abcd" ;
$a[3] = "b" ;

($f) = grep {$a[$_] =~ /^abc$/} (0..$#a);# ? 1 : 0;
if ( $f ) {
    print "YES\n" ;
}
1;
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Therefore can be greatly simplified to become:

#!/usr/bin/perl -w
use strict;
my (@f) = qw|a ab abc abcd|;

if (grep {/Zabc/} @f){
    print 'YES!';
}
1;
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Also, if you read grep's perldoc, you will learn more about it:

 In scalar context, returns the number of times the
 expression was true.
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