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BEGIN { $sep= '' }
if(  '' eq $sep  &&  1 < @F  ) {
    ( $sep )= /\Q$F[0]\E(.*?)\Q$F[1]\E/;
}

my $pos= 0;
for my $f (  0..$#F  ) {
...
    substr( $_, $-[0], $+[0]-$-[0], $G[$f] );
}
pos($_)= 0;