A "more Perlish" way to do this is something like (untested):

my @file_line_contents = ...;

my @line_lengths = map length($_), @file_line_contents;
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See map.

Update: Or if  @file_line_contents is likely to be more than 45 elements and you want the length array to be limited to 45 (untested):

my @file_line_contents = ...;

my @line_lengths = map length($_), @file_line_contents[ 0 .. 44 ];
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See Slices in perldata.