BEGIN {
    my $code= '...some work-around for not having GD...';
    if( eval { require GD } ) {
        $code= '...code that uses GD...';
    }
    eval "sub mySub { $code }; 1 "
      or die "$@";
}

##</code><code>##

package Pretend;
use base qw(Exporter);
our @EXPORT_OK= qw( sorter hasher );
sub sorter(&@);
sub hahser(\%);
#[...]

##</code><code>##

my $got_Pretend;
BEGIN {
    if(  $got_Pretend= eval { require Pretend }  ) {
        Pretend->import(  qw( sorter hasher )  );
    } else {
        require WorsePretendWorkAround;
        WorsePretendWorkAround->import(
           qw( comparer worse_hasher )  );
    }
}
#[...]
    if(  $got_Pretend  ) {
        @list= sorter { $_[0] <=> $_[1] } hasher %hash;
    } else {
        @list= sort \&comparer worse_hasher(\%hash);
    }

##</code><code>##

    if(  $got_Pretend  ) {
        @list= sorter( sub { $_[0] <=> $_[1] }, &hasher(\%hash) );
    } else {
        @list= sort \&comparer worse_hasher(\%hash);
    }