use strict;
use warnings;

for ( 'NA12345', 67890 )
{
    my $id = $_;
    print "$id -> ";
    $id =~ s/^NA//;
    print "$id\n";
}

__END__
NA12345 -> 12345
67890 -> 67890

##</code><code>##

next unless $line =~ m{^(\S+) (\d+) (.*)};

##</code><code>##

use strict;
use warnings;

for( 'string1 NA12345 other stuff',
      'string2 67890 more stuff' )
{
    if( $_ =~ m/^(\S+) ((?:NA)?\d+) (.*)/ )
    {
        print "matched: $2\n";
    }
}

__END__
matched: NA12345
matched: 67890

##</code><code>##

use strict;
use warnings;

for( 'string1 NA12345 other stuff',
      'string2 67890 more stuff' )
{
    my @elements = split( /\s/, $_, 3 );
    print( '[', join( '][', @elements ), "]\n" );
}

__END__
[string1][NA12345][other stuff]
[string2][67890][more stuff]