No, that's not quite right.

In the first example, change increment() to return $data instead of undef, and then you can call error() without getting an uninitialized value error.

In the second example, simply give $data a default value, call error() before your while block, and you'll see that it prints.

Here's the code I used for an example:

#!/usr/bin/perl -w

use strict;
my $data;
my $ERROR = 0;

# error();    # uncomment this to print $data before the loop

while ( (defined $data++) && everything_is_okay() ) {
    print "looping for the $data time.\n";
    if ( $data > 5 ) {
        $ERROR++;
        error();    # print $data if there's an error
    }
}
sub everything_is_okay {
    if ( $ERROR ) {
        print STDERR "ERROR LEVEL $ERROR\n";
        exit $ERROR;
    }
    return 1;
}
sub error {
    print $data;
}
[download]

I put the defined check in there so the loop will get past the first iteration. $data++ (when $data is not initialized) evaluates to zero:

my $data;
print $data++;
[download]

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