use warnings;
use strict;

my @routers = ( "R1", "R2", "R3" );
my %if = (
	R1 => { R1R0  => 1, R1R6  => 2, },
	R2 => { R2R0  => 3, R2R6  => 1, },
	R3 => { R3R61 => 2, R3R62 => 3, },
);

for my $r (@routers) {
	for my $i (keys %{ $if{$r} }) {
		print "Router $r Interface $i\n";
	}
}

__END__

Router R1 Interface R1R0
Router R1 Interface R1R6
Router R2 Interface R2R0
Router R2 Interface R2R6
Router R3 Interface R3R61
Router R3 Interface R3R62

##</code><code>##

use warnings;
use strict;

my @routers = ( "R1", "R2", "R3" );
my %interfaces =  (
	R1R0  => 1, R1R6  => 2,
	R2R0  => 3, R2R6  => 1,
	R3R61 => 2, R3R62 => 3,
);

for my $r (@routers) {
	my @ifs = grep {/^\Q$r\E\D/} keys %interfaces;
	for my $i (@ifs) {
		print "Router $r Interface $i\n";
	}
}