#!/usr/bin/perl

use warnings; use strict;

# open filea and parse all id strings. 
# Add id strings as keys to %wanted array.
my %wanted;
{
    open my $file, '<', "filea" 
      || die "failed to open filea : $!";
    while( <$file>)
    {
        chomp;
        @ids = split( /\s+/, $_);
        $wanted{ $_ }++ for @ids;
    }
    close $file;
}

#read fileb, parse lines of the form "id <whitespace> letters" 
#and print lines that match the id strings from filea.
{
    open my $file, '<', 'fileb'
      || die "failed to open fileb : $!";
    while (<$file>)
    {
        chomp;
        my ($id, $letters) = split( /\s+/, $_);
        print "$id\t$letters\n" if $wanted{$id};
    }
}

#OR

#read fileb, parse lines of the form "id <newline> letters" 
#and print lines that match the id strings from filea.
{
    open my $file, '<', 'fileb'
      || die "failed to open fileb : $!";
    while (<$file>)
    {
        my $id = $_;
        my $letters = <$file>;
        chomp($id);
        chomp($letters);
        
        print "$id\t$letters\n" if $wanted{$id};
    }
}
__END__
FileA:
1DWK 2RFK 
4ERH

FileB:
1DWK HRSDKKDAHJKLSDLDLLJDGHDFJJE
4ERH DFSKFHADFSBVHFWIHFWJBFS
2RFK DADUHRQWERKBNJAIJDLAJDKAKDNAKDJKSADJKAHDJASHRWEUB

FileB (alternate):
1DWK
HRSDKKDAHJKLSDLDLLJDGHDFJJE
4ERH
DFSKFHADFSBVHFWIHFWJBFS
2RFK
DADUHRQWERKBNJAIJDLAJDKAKDNAKDJKSADJKAHDJASHRWEUB