<>;
if ( /^aaa/ ) {
    DataFatal( "problem-description, for $_" );
}
chomp;
my $file = $_;
if ( /okregexp/ ) {
   #do something
}
else { #matches neither!
   # this case wasn't mentioned, by the way!
}

sub DataFatal { # fatal messaging.
    print STDERR "*** DATA FATAL ***\n+++ " . shift . " +++\n";
    exit 1;
}