perl - doing a substitution until certain condition is true -

i'm trying edit text using perl. need make substitution substitution cannot applied once specific word found in text. so, imagine want substitute "hello" forms "goodbye", substitution cannot applied once word "foo" found.

i tried this:

use warnings; use strict; $/ = undef; $filename = shift; open f, $filename or die "usa: $0 filename\n"; while(<f>) { {s/hello/goodbay/} until (m{foo}); print; } close f; 

but, result, first "hello" of text changed.

any suggestion?

trying think efficient. should 1 of following:

s{^(.*?)(foo|\z)}{    $s = $1;    $s =~ s{hello}{goodbay}g;    $s.$2 }se; print; 

or (same above, requires 5.14+)

s{^(.*?)(foo|\z)}{ s{hello}{goodbay}gr . $2 }se; print; 

or

my $pos = /foo/ ? $-[0] : length; $s = substr($_, 0, $pos, ''); $s =~ s{hello}{goodbay}g; print($s); print; 

both work if foo isn't present.

this solution uses less memory:

# assumes foo present # (though expanded handle # assumes foo isn't regex pattern. local $/ = "foo"; $_ = <$fh>; chomp; s{hello}{goodbay}g; print;  print $/;  local $/; print <$fh>;