my $GREP_STR = "123"; ## a global var

print GREP_FH "My line with 123\n"; ## anywhere in the code
print GREP_FH "My line with 456\n";

##</code><code>##

&myprint("My line with 123\n");
&myprint("My line with 456\n");

##</code><code>##

$GREP_STR = '123';

print STDOUT "Header - always print this\n";
print GREP_FH "My line with 123\n";    ## printed at the terminal
print GREP_FH "My line with 456\n";    ## not printed