For the benefit of other monks who might be trying their hand at this:

use Test::More;

my %output_for
    = ( '215000007801' => '21507801',
        '300000324002' => '30324002',
        '890000457651' => '89457651',
        '210004563401' => '214563401',
        '201045139158' => '20145139158', );

plan tests => scalar keys %output_for;

sub solution {
    # (this doesn't work)
    $_[0] =~ s{ \A ([^0]+ 0) ([^0]*) 0+ ([^0]+0) }{$1$2$3}x
#        ||
#    $_[0] =~ s{ \A ([^0]+) 0+ ([^0]) }{$1$2}x;
}

while ( my ($input, $correct_output) = each %output_for ) {
    my $orig_input = $input;
    solution( $input );
    is( $input, $correct_output, "Solved '$orig_input'" );
}
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