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

my $str1 = <<STR;
col1    col2    col3
1    4    7
2    5    8
3    6    9
STR
my $str2 = <<STR;
col1    col2    col3
1    44    77
2    55    88
3    66    99
STR

for my $spec ([$str1, qw(col1 col3)], [$str2, qw(col3)]) {
    my ($file, @wantedCols) = @$spec;

    print "@wantedCols\n";

    open my $fIn, '<', \$file;
    
    my $index = 0;
    my %fileCols = map{$_ => $index++} split /\s+/, <$fIn>;
    my @slice = map{exists $fileCols{$_} ? $fileCols{$_} : ()} @wantedCols;
    while (<$fIn>) {
        chomp;
        print join (' ', (split /\s+/)[@slice]), "\n";
    }
}

##</code><code>##

col1 col3
1 7
2 8
3 9
col3
77
88
99