Your line $line="abc.xyz@lmnop.com"; is using double-quotes so you are trying to interpolate the array @lmnop into the string. Use single-quotes to suppress interpolation.

knoppix@Microknoppix:~$ perl -E '
> $line = q{abc.xyz@lmnop.com};
> say $line;
> $line =~ s{(?=\@)}{\\};
> say $line;'
abc.xyz@lmnop.com
abc.xyz\@lmnop.com
knoppix@Microknoppix:~$
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I hope this is helpful.