my $from = "abc1\@gmail.com";   # double quotes
my $to   = 'abc3\@gmail.com';   # single quotes

print "from: $from";
print "to:   $to";

__END__
from: abc1@gmail.com
to:   abc3\@gmail.com           # !!

##</code><code>##

#!/usr/bin/perl -l
use Email::Address;

@addresses = Email::Address->parse('abc3@gmail.com');
print "ok     : @addresses";

@addresses = Email::Address->parse('abc3\@gmail.com');
print "not ok : @addresses";

__END__
ok     : abc3@gmail.com
not ok :