Slice files into sections by regex

fileslice.pl

#!/usr/bin/perl
use warnings;
use strict;

my $REGEX = shift @ARGV;
my $SourceFile = shift @ARGV;
my $OutputFormat = shift @ARGV;

my $FileI = 0;
open(SRC, '<', $SourceFile) or die "Unable to open $SourceFile: $!";
my $OutF = undef;
while (<SRC>) {
        if (my @captures = ($_ =~ /$REGEX/)) {
                close($OutF) if (defined($OutF));
                printf "Opening %s\n", formatfilename($OutputFormat, $FileI, @captures);
                open($OutF, '>', formatfilename($OutputFormat, $FileI++, @captures));
        }
        unless (defined($OutF)) {
                printf "Opening %s\n", formatfilename($OutputFormat, $FileI);
                open($OutF, '>', formatfilename($OutputFormat, $FileI++));
        }
        print $OutF $_;
}

sub formatfilename {
        my $Format = shift;
        my $i = 0;
        while (@_) {
                my $arg = shift(@_);
                $Format =~ s/%$i/$arg/g;
                $i++;
        }
        return $Format;
}