In case you are unfamiliar with using Perl directly on the command line the -pi.bak part makes a backup file with a .bak extension before the -e (for execute) executes the 's/this/that/g' on every line of myfile.pl. For a more permanent solution just write a little script like this:

#!/usr/bin/perl -w -i.bak
use strict;
while (<>) {
    s/this/that/g;
    print;
}
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If you call this script say convert.pl you use it like this:

C:\>type convert.pl
#!/usr/bin/perl -w -i.bak
use strict;
while (<>) {
    s/this/that/g;
    print;
}

C:\>type this.txt
this
this
this
this

C:\>perl convert.pl this.txt

C:\>type this.txt
that
that
that
that

C:\>type this.txt.bak
this
this
this
this

C:\>
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Just change this and that to whatever you want. For Mac that will be \r and \n respectively.