my ($pattern,$id) = (split /\s+/, $_)[0,4];

##</code><code>##

my ($pattern, $id) = (split /\s+/)[0, 2];

##</code><code>##

if ( $_ =~ m/^$pattern/ ) {

##</code><code>##

...
my $pattern_to_find = '4567';
...
while (<MAST>) {
    my ($pattern, $id) = (split /\s+/)[0, 2];
    ...
    if ($pattern eq $pattern_to_find) {
        ...