One way is to use the substitution operator, s///. The following code will only keep digits and the decimal point. You should be able to modify the character class to keep other things, if needed.

use strict;
use warnings;

for (qw(12.34% 0.05lb 999)) {
    my $s = $_;
    $s =~ s/[^\d.]//g;
    print "$s\n";
}

__END__
12.34
0.05
999
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