When you have an escape character ('\') in your string the way you assign it to a variable will affect the result. If you assign using double quotes then you need to escape the escape character in your string, just like you do in the regular expression, and this is what the error is telling you. If you use single quotes it will work without escaping:

my $string;

print "Double quotes\n";
$string = "LoadStore.SGE.\sr1_addr_reg_tmp_reg[0]";
match();

print "Double quotes with escape\n";
$string = "LoadStore.SGE.\\sr1_addr_reg_tmp_reg[0]";
match();

print "Single quotes\n";
$string = 'LoadStore.SGE.\sr1_addr_reg_tmp_reg[0]';
match();

sub match {
    if ( $string =~ m/LoadStore\.SGE\.\\.r._addr_reg_tmp_reg/ ) {
        print "match\n";
    }
    else {
        print "no match\n";
    }
}
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Output

Double quotes
no match
Error: Unrecognized escape \s passed through

Double quotes with escape
match

Single quotes
match
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