> > #############################
> > use strict;
> > use warnings;

> > my $var = "''''''''''''";

> > my $p_pulse = q{./\\};
> > my $n_pulse = q{'\\/};

> > my $knt = 1;

> > print $var;
> > $var =~ s/^((...){$knt}).../$1 . (($1 =~ m{'}) ? $n_pulse : $p_pulse)/
> > e;       ## <---- offending line
> > print $var;
> > ##########################

> > when I run the above piece of perl code i get the following warnings:
> >       Use of uninitialized value in concatenation (.) or string at
> > line 12.
> > And when i change the $var to "..........." it works fine.

> > what could be the problem here?

> The $1, $2, $3 <etc> variables are set after every successful pattern
> match.  In your replacement, you are doing a pattern match.  That
> pattern match succeedes, because the current value of $1 does indeed
> match m{'}.  That successful match causes $1 to be set to undef,
> because that pattern match doesn't have any capturing parentheses.  So
> it is the first $1 listed in your replacement - the one that's
> concatenated to the result of the ?: operator - that's undefined.

> It worked fine when you changed $var to all periods because then the
> inner pattern match was not successful and so $1 wasn't changed.

> You can fix this by saving off the value of $1 in your replacement, as
> follows:

> $var =~ s/^((...){$knt}).../my $save = $1; $save . (($save =~ m{'}) ?
> $n_pulse : $p_pulse)/e;

> Hope this helps,
> Paul Lalli