Perldoc is a great resource. Check out perldoc perlretut (tutorial) and perldoc perlre (all the details). Module Regexp::Debugger is also great for visualizing how the matches are happening.
Here’s one possible implementation, based on the scant details you provided.
#!/usr/bin/env perl
use 5.014;
use warnings;
my $data="mnttab 0K 0K 0K 54% /etc/mnttab";
my ( $percent ) = $data =~ /(\d+)%/;
say $percent;
solved Want to filter the percentage from my output [closed]