use strict; use warnings; use feature 'say';

while ( my $line = <DATA> ) {
    chomp $line;
    say replace( $line, 'SUBSTR', 'FOO' );
}

sub replace {
    my ( $input, $wanted, $replacement ) = @_;
    if ($input =~ m/$wanted/) { return $replacement;} 
    else { return $input; } 
        
  
}

__DATA__
path/to/some/file
path/to/some/other/file
path/to/SUBSTR/file #replace entire line if SUBSTRING is found
path/to/file

##</code><code>##

path/to/some/file
path/to/some/other/file
FOO
path/to/file