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
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