You can look at the symbol table of a package for code entries. E.g. for "main"

no strict 'refs';
for ( keys %{"main::"} ) {
  print "$_\n" if defined *{"main::$_"}{CODE};
}
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If you want to know subroutines of everything loaded, then you can use the %INC hash, convert back to module names, and search through all of them for subroutines:

my @subs;
for my $mod ( keys %INC ) {
    $mod =~ s!/!::!g;
    $mod =~ s/.pm$/::/;
    {
        no strict 'refs';
        for ( keys %{$mod} ) {
            push @subs, $mod . $_ if defined *{$mod . $_}{CODE};
        }
    }
}
print "$_\n" for @subs;
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(Note that this won't find subroutines that are explicitely loaded into a package that isn't named the same as the file that was required or used.)