while (<>) {
    my ($prefix, $tail) = /(\S+) (.*)/;
    
    print "\n" if $currPrefix && $currPrefix ne $prefix;
    print "$prefix $tail";
    $currPrefix = $prefix;
}

##</code><code>##

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

my $in = *STDIN;
my $out = *STDOUT;

if (@ARGV == 2) {
    open $in, '<', $ARGV[0] or die "Can't open '$ARGV[0]': $!\n";
    open $out, '>', $ARGV[1] or die "Can't create '$ARGV[1]': $!\n";
}

my $currPrefix;

while (<$in>) {
    my ($prefix, $tail) = /(\S+) (.*)/;

    print $out "\n" if $currPrefix && $currPrefix ne $prefix;
    print $out "$prefix $tail";
    $currPrefix = $prefix;
}

##</code><code>##

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

my $currPrefix;

$^I = '.bak';

while (<>) {
    my ($prefix, $tail) = /(\S+) (.*)/;

    print "\n" if $currPrefix && $currPrefix ne $prefix;
    print "$prefix $tail";
    $currPrefix = $prefix;
}