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

my $i = 0;
my @out_files;
foreach ( qw( op0 op1 op2 op3 ) ) {
    unless ( open $out_files[ $i ], '>', $_ ) {
        die "Cannot open $_ for writing: $!\n";
    }
    $i++;
}

while ( <> ) {
    foreach my $out ( @out_files ) {
        print $out $_;
    }
}

##</code><code>##

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

my $line;
my %out = map { $_ => undef } qw( op0 op1 op2 op3 );
( open $out{ $_ }, '>', $_ or die "can't: $!\n" ) for keys %out;
while ( $line = <> ) { print $_ $line for values %out; }

##</code><code>##

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

my @out = qw( op0 op1 op2 op3 );
my %out = map { $_ => undef } @out;
for ( @out ) { open $out{ $_ }, '>', $_ || die "Can't: $!\n" }
while ( my $l = <> ) { print $_ $l for values %out }