The question is not entirely obvious to me (see I know what I mean. Why don't you?), but perhaps you would like to use a dispatch table, which is implemented with a hash of subroutine references:

#!/usr/bin/perl -w
use strict;

my %dispatch = (
    f11 => \&function11,
    f12 => \&function12,
    f21 => \&function21,
    f22 => \&function22,
);


sub function11 {
    return "Function 1-1\n";
}

sub function12 {
    return "Function 1-2\n";
}

sub function21 {
    return "Function 2-1\n";
}

sub function22 {
    return "Function 2-2\n";
}

for my $i (1..2) {
    for my $j (1..2) {
        print $dispatch{"f$i$j"}->();
    }
}
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See perlref for more information on sub refs.