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 $line;'
abc.xyz@lmnop.com
abc.xyz\@lmnop.com
knoppix@Microknoppix:~$

[download]

I hope this is helpful.

Cheers,

JohnGG

In reply to Re: how to substitute '@' in line by johngg
in thread how to substitute '@' in line by vishy_acts

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