use Regexp::Common qw /delimited/;
my $text = q~
qqq <img   src  = "src"  >
sss <img   src='src' >

~;
$text =~ s~
         img \s+ src
        \s* \= \s*
        (?: $RE{delimited}{-delim=>'"'} |
            $RE{delimited}{-delim=>"'"}
        )
         ~bongo~sigx;
print $text,$/,$/;
__END__

qqq <bongo  >
sss <bongo >