Encode the knowledge of what substitions are possible as data instead of code and let the regex engine take the strain.
#! perl -slw
use strict;
my %subs = (
100 => '50 50',
50 => '20 20 10',
20 => '10 10',
10 => '5 5',
5 => '2 2 1',
2 => '1 1',
);
my $re_conversion = '('
. join( '|', sort{ length $b <=> length $a } keys %subs )
. ')';
my $input );
do {
printf 'Denomination to change[100|50|20|10|5|2]: ';
chomp( $input = <STDIN> );
} until exists $subs{ $input };
print $input while $input =~ s[$re_conversion][$subs{ $1 }];
__END__
[11:58:38.32] P:\test>406984
Denomination to change[100|50|20|10|5|2]: 20
10 10
5 5 10
2 2 1 5 10
1 1 2 1 5 10
1 1 1 1 1 5 10
1 1 1 1 1 2 2 1 10
1 1 1 1 1 1 1 2 1 10
1 1 1 1 1 1 1 1 1 1 10
1 1 1 1 1 1 1 1 1 1 5 5
1 1 1 1 1 1 1 1 1 1 2 2 1 5
1 1 1 1 1 1 1 1 1 1 1 1 2 1 5
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
"Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon